Circulatory System: Embryonic Veins
The vitelline circulation is the flow of blood between the embryo and the yolk sac through a collection of vitelline arteries and veins that pass within the yolk stalk (Figure 30.1).
The vitelline arteries are branches of the dorsal aortae, and most of them degenerate in time. Those that remain fuse and form the 3 unpaired ventral arterial branches of the aorta that supply the gut: the celiac trunk, superior mesenteric artery and inferior mesenteric artery.
The vitelline veins will give rise to the hepatic portal vein and the hepatic veins of the liver.
The umbilical circulation is the flow of blood between the chorion of the placenta and the embryo. The umbilical arteries carry poorly oxygenated blood to the placenta and the veins carry highly oxygenated blood initially to the heart of the embryo (Figure 30.1), and later into the liver when it forms (see Figure 31.1). The right umbilical vein is lost arou ng only the left to carry blood from the placenta.
The formation of the ductus venosus during the foetal period causes about half of the blood from the umbilical vein to flow directly into the inferior vena cava, bypassing the liver (Figure 31.1). This, with other mechanisms, preferentially shunts highly oxygenated blood to the foetal brain.
Of the umbilical arteries only the proximal portions persist as parts of the internal iliac arteries and superior vesical arteries in the adult. The distal portions do not remain as arteries but become the medial umbilical ligaments. The umbilical vein becomes the ligamentum teres, passing from the umbilicus to the porta hepatis in the adult (see Chapter 31).
The common cardinal veins initially form an H‐shaped structure, with the horizontal bar being the sinus venosus that links the major veins and the atrium of the early heart tube (Figure 30.2). The left and right anterior (or superior) branches drain blood from the head and shoulder regions and the posterior (or inferior) branches drain from the abdomen, pelvis and lower limbs.
At 6 weeks a subcardinal vein arises on either side of the embryo caudal to the heart and anastomoses with the posterior cardinal veins (Figure 30.3). The subcardinal veins also form an anastomosis with each other anterior to the dorsal aortae, and tributaries are sent into the developing limbs. The right subcardinal vein joins vessels of the liver. Similarly, at 7 weeks supracardinal veins form and link to the posterior cardinal veins (Figure 30.3).
The posterior cardinal veins degenerate, although the most caudal parts continue as a sacral venous plexus and later as the common iliac veins.
An important junction between the right supracardinal and right subcardinal vein forms and both will become sections of the inferior vena cava (IVC). Parts of the right posterior cardinal veins, common, subcardinal and supracardinal veins also contribute. A shift towards the right side occurs, with degeneration of venous structures on the left side and the formation and enlargement of the inferior vena cava on the right (Figure 30.4).
Similarly, the degeneration of much of the left anterior cardinal vein gives a shift to the right side as the right anterior cardinal vein forms part of the superior vena cava (SVC) and the right brachiocephalic vein (Figure 30.4). An anastomosis between the 2 anterior cardinal veins persists as the left brachiocephalic vein.
The right supracardinal vein becomes much of the azygos vein, and the left supracardinal vein forms part of the hemiazygos vein and the accessory hemiazygos veins (Figure 30.4). Branches from the subcardinal vein network form renal, suprarenal and the gonadal veins.
The formation of the venous system is somewhat variable and complicated, and can give rise to variations in adult SVC and IVC anatomy. The hepatic section of the IVC may fail to form, for example, and blood instead flows back to the heart through the azygos and hemiazygos veins from the inferior parts of the body (azygos continuation). Persistence of supracardinal veins can leave double inferior vena cavae, and persistence of the left anterior cardinal vein can give double SVC. In this case the right anterior vena cava may even dege left SVC. These variations are not common.